It is common practice to add cement dispersants such as plasticizers and superplasticizers to concrete and mortars to render such more fluid and/or to increase strength by reducing the amount of mix water required to obtain a desired fluidity. It is well-established, however, that when the dosage rate of a dispersing agent is progressively increased, an increase in the initial workability or yield stress (or “slump”) of the cementitious mixture is usually obtained. Furthermore, based on the particular type of dispersant employed, the workability retention of the mixture is extended for some period of time.
The workability of the concrete or mortar mixture is referred to by using the term “slump,” which is a property that can be quantitatively measured by the extent to which fresh concrete or mortar flows when demolded or removed from an inverted cone on a flat surface.
Normally, increasing the dosage of certain dispersing agents or superplasticizer to an excessive amount to achieve longer slump life causes the initial workability to increase such that the mixture is no longer stable. This loss of stability is manifested in terms of severe segregation of sand and/or stone aggregate constituents from the wet cement paste in the concrete or mortar. Furthermore, such excessive dosages of the dispersing agent can cause undesirable extended setting times of the treated cementitious mixtures.
Thus, an objective of the present invention is to provide methods and compositions which allow a concrete producer to extend the slump life of the concrete by varying the dosage of the dispersant formulation in the concrete mixture, whereby the initial workability of the mixture, once a certain desired slump range has been achieved, changes to a significantly far less degree versus the accompanying slump retention, thus avoiding the problem of segregation of the concrete components.
Another objective of the present invention is to provide the ability to add variable amounts of a dispersing agent or superplasticizer that is operative to increase workability or slump retention of the concrete mixture at later times (e.g., for the period of 30 minutes to three hours after cement and aggregate(s) are mixed together to form the hydratable concrete or cementitious mixtures).